The continuing purpose of this project is the study of Tyzzer's disease. The topics of present interest are: (1) characterization of the causative agent, Bacillus piliformis, of which we presently have isolants from four animal species (rabbit, horse, rat and mouse) in pure culture; (2) the serological response of the host to B. piliformis infection and the serological relationship between isolants from different animal species using the complement fixation test and fluorescent antibody techniques; (3) the pathogenesis of Tyzzer's disease in various animal species; and (4) the epidemiology of Tyzzer's disease, the goal being to identify primary sources of the infection.